Retrieve variable value in another class

Question

I'm using kivy + kivyMD and pyrebase, to do the login game or return from Firebase in a "user" variable, I want to use this variable to reconnect at any time inside the Login class.

Or problem is when I do or login correctly it directs me to another class and then I can't recover this value anymore.

Below is an example that is minimalist to guide me that can help me:

On line 117 I have a variable with any value. I would like to use this variable in a class that I HAVE to create in the fabric "scr 1" in line 44 (Note that I still do not create a class because I am giving a minimalist example)

I would like to obtain this value through the python code and not in the kv file.

reduced example